This invention relates to a method for mechanically deaerating aerated bitumen froth to reduce its air content to render it pumpable. More particularly it relates to mechanically shearing aerated bitumen froth by either passing the froth through a confining passageway and shearing the froth with an impeller while it is in the passageway or temporarily retaining the aerated froth in a tank and circulating it repeatedly through a pump.
Oil sand, as known in the Fort McMurray region of Alberta, Canada, comprises water-wet sand grains having viscous bitumen flecks trapped between the grains. It lends itself to separating or dispersing the bitumen from the sand grains by slurrying the as-mined oil sand in water so that the bitumen flecks move into the aqueous phase.
For the past 25 years, the bitumen in McMurray sand has been commercially recovered from oil sand using a hot water process. In general terms, this process involves mixing surface-mined oil sand with heated water, steam and sodium hydroxide in a rotating tumbler to initially disperse the bitumen to form a slurry that has a temperature of about 80xc2x0 C. The slurry is further diluted with heated water and then introduced into a primary separation vessel (PSV) where the more buoyant bitumen particles float to the surface to form a froth. This froth overflows the vessel wall and is received in a launder extending around the PSV""s rim. The product is commonly called xe2x80x9cprimary frothxe2x80x9d and typically comprises 66% bitumen, 9% solids and 25% water. It is usually at a temperature of about 75xc2x0 C. The primary froth also contains approximately 30 vol. % air.
The primary froth typically is deaerated to about 13 vol. % air, at which point it is capable of being pumped by centrifugal pumps through a pipeline to the froth treatment plant. Deaeration is achieved by feeding the bitumen froth by gravity through a deaeration tower having vertically spaced sheds. The froth forms thin layers on the sheds and is countercurrently contacted with steam, to both heat and deaerate the froth. The deaerator circuit is similar to that described in U.S. Pat. No. 4,116,809, issued to Kizior on Sep. 26, 1978.
A recent development in the recovery of bitumen from oil sand involves a low energy extraction process (LEE process). The LEE process is not in the public domain but is in the process of being patented. The LEE process can be summarized as follows:
locating a mine remote from the upgrading refinery;
mixing the oil sand with heated water at the mine site to produce a
pumpable, dense, low temperature slurry having a density in the range
1.4 to 1.65 g/cc and temperature in the range 20 to 35xc2x0 C.;
pumping the slurry through a pipeline to an extraction site, the pipeline
being of sufficient length so that the slurry is conditioned for flotation;
aerating the slurry and diluting it with water as it moves through the pipeline; and
delivering the aerated diluted slurry into a primary separation vessel (PSV) and producing bitumen froth (xe2x80x9cprimary frothxe2x80x9d). The buoyant bitumen froth floats to the surface of the PSV where it overflows the vessel""s walls into a launder that recovers the overflowing bitumen froth. The LEE primary froth obtained from medium grade oil sand typically comprises 60% bitumen, 29% water and 11% solids and has an air content of approximately 50 vol. %. Depending on the oil sand and the experimental conditions, LEE froth air contents have been measured between 28 to 72 vol. %. As was the case with the bitumen froth obtained from the hot water process, the froth obtained using the LEE process must be deaerated to a reduced air content (preferably less than 10%) to minimize impact on pump performance when the froth is pumped by centrifugal pumps through the pipeline to the upgrading facility.
At the applicant""s commercial operation, the current site for low energy extraction is 35 km away from the main processing plant and its utilities. Therefore, use of the conventional deaeration tower with steam to deaerate the bitumen froth would be very expensive for the following reasons:
it would be expensive to move the steam from the main plant through a long pipeline to the extraction site in cold weather; and
alternatively, it would be expensive to build a utility plant at the extraction site and heat and treat the water at that point. Steam production requires clean water and therefore the water must be chemically treated before it can be reused. In light of the above, an alternate process for deaerating low energy froth was pursued using mechanical break-up or shearing.
There are two concerns that need to be addressed when designing a mechanical shearing process for use with a unique feed stock such as bitumen froth. Firstly, there is a concern that if the mechanical shearing is too vigorous, the air bubbles will actually break up into even smaller air bubbles. It is known in the art that it is more difficult for smaller bubbles to move through the bitumen matrix and reach the surface where they can break out.
Second, there is a concern that mechanical shearing will cause the water and solids in the bitumen froth to emulsify. If emulsification occurs, it makes it more difficult for the downstream centrifuges to carry out their separation work, that is, to separate the solids and water from the bitumen.
Taking into account the above concerns, two alternative mechanical shearing processes have been developed which are specifically tailored to be used with low temperature (20 to 45xc2x0 C.), viscous, solids-containing bitumen froth.
The present invention is based on the discovery that mechanical shearing is effective to deaerate bitumen froth sufficiently so that it is pumpable and thus can be propelled through a pipeline. The discovery is particularly useful because it has been shown to work with LEE bitumen froth, which typically has a temperature between 20 to 45xc2x0 C. and therefore is quite viscous. It was not predictable that mechanical shearing would be effective to reduce the air content in such froth to less than 10 vol. %, preferably about 6 vol. %. The air content in deaerated froth has to be sufficiently low in order for the froth to be pumpable for pipeline purposes. We have demonstrated that two distinct ways of mechanically shearing the froth will reduce its air content to the desired level. More particularly:
passing the froth through a confining passageway and shearing the froth with an impeller while it is in the passageway; or
temporarily retaining the aerated froth in a tank and circulating it repeatedly through a pump;
will each serve to successfully deaerate the froth so that it is pumpable.
So, in one aspect the invention provides a method for deaerating bitumen froth produced by flotation in a primary separation vessel and recovered therefrom, comprising mechanically shearing the froth to reduce its air content sufficiently so that the deaerated froth can be pumped through a pipeline.
Having ascertained that mechanically shearing LEE bitumen froth will work to deaerate it as required, we have combined it with the LEE process to provide a novel method for recovering deaerated bitumen froth from oil sand containing bitumen comprising:
dry mining the oil sand;
mixing the as-mined oil sand with heated water to produce a slurry having a density in the range 1.4 to 1.65 g/cc and a temperature in the range 20 to 35xc2x0 C.;
pumping the slurry through a pipeline for sufficient distance to condition the slurry;
adding flood water and air to the slurry, preferably as it moves through the pipeline, to produce a diluted, aerated slurry;
introducing the product slurry into a primary separation vessel and temporarily retaining it therein under quiescent conditions while simultaneously preferably injecting hot underwash water just below the forming froth to raise its temperature and venting excess air out of the PSV feedwell, to produce aerated bitumen froth; and
recovering the froth and mechanically shearing it to deaerate it sufficiently so that the deaerated froth can be pumped through a pipeline.